I run my XM's(generally) 6hpd and my single DE 4hpd... they will last much longer and it's been proven that over 4hpd of metal halide is overkill for the benefit of the zoozanthellae. A longer lighting period does, however seem to enhance the colors of the corals trying to protect themselves from the prolonged photoperiod(actually the zoozanthellae, also). About 2 months before our reef tour, I slowly adjust my lighting period for the XM's to 8hpd and my single DE to 6 hpd.